This disclosure relates to batteries including electrochemical cells. Alkaline manganese dioxide cells have been predominantly used as primary batteries. However, the one-time use of primary batteries results in large material wastage as well as undesirable environmental consequences. Also, potential economic losses can arise due to the significant imbalance between the energy that is required to manufacture these cells compared to the energy that can be actually stored. As a consequence, there is a clear advantage to convert primary cells to rechargeable or secondary cells.
Manganese dioxide has proven recalcitrant to this necessary conversion due to fundamental problems with its crystal structure and side reactions which result in products that are not amenable to charge-discharge cycling. Efforts to develop the zinc-manganese dioxide battery system date back more than forty years, with many unsuccessful attempts made to commercialize it. Secondary (rechargeable) alkaline batteries have recently been marketed using a technology disclosed in U.S. Pat. No. 4,957,827. These rechargeable alkaline batteries require proprietary chargers that improve cycleability (U.S. Pat. No. 7,718,305). However, their lifetime is limited due to fall off in capacity at the high depths of discharge that are of interest in many commercial applications. Due to these limitations, rechargeable alkaline batteries have not yet witnessed widespread adoption.
The theoretical capacity that a manganese dioxide crystal can discharge is 617 mAh/g, which is based on the incorporation of two electrons in the redox reaction. To access this capacity during the discharge process, the manganese dioxide crystal must undergo stressful phase transformations and chemical reaction steps that may lead to its eventual breakdown and loss of rechargeable material. To control these lattice dilations and chemical transformations, the cycled capacity has usually been limited to 5 to 10% of the overall capacity. Wroblowa et al. (EP0138316A1 and U.S. Pat. No. 4,451,543) found that synthesizing the birnessite-phase of manganese dioxide, and incorporating bismuth and lead in the crystal structure through physical or chemical means imparted rechargeability characteristics to the manganese dioxide material. In some cases, they were able to obtain up to 80-90% of the second electron capacity. Yao (U.S. Pat. No. 4,520,005) found a way of incorporating bismuth and lead in the birnessite-phase of the manganese dioxide in a single step reaction. Yao's method was a variation of the original synthesis method by Wadsley (JACS, Vol 72, 1781, 1950). Rechargeable Battery Corporation (U.S. Pat. Nos. 5,952,124 and 5,156,934) developed methods for synthesizing oxides or hydroxides of bismuth coated on manganese dioxide and heating nitrates of bismuth and manganese to create a phase of bismuth-manganese dioxide. The prior art shows that bismuth plays a role in lattice stabilization and in avoiding the electrochemical inactive phase of hausmannite (Mn3O4) during cycling. However, none of the prior art could develop high cycle life with good reliability and reproducibility. Extensive testing indicates that within a few charge-discharge cycles the depth of discharge obtainable falls off rapidly with a large loss of capacity. Also, it was found that the high cycle life obtained in the publications relied on cycling the material potentiodynamically, a cycling protocol that cannot be used in real world applications, rather than galvanostatically which is a preferred protocol to cycle batteries in real world applications. Potentiodynamic cycling is an experimental method in electrochemistry to test the chemical reactions taking place on the electrode, which is not the way that actual batteries operate. Prior arts and literature publications have relied on this type of cycling protocol to show high cycle life, however, on galvanostatic cycling there is rapid loss in capacity that leads to the immediate failure of the battery. Also, the prior art has relied on making electrodes with excessive carbons (about 10 times more than MnO2 loading), to show high cycle life. However, batteries containing electrodes with a 10 times excess of carbon are not economically viable and have very poor energy density that would be impractical in any real world application. Extensive tests have shown that electrodes containing 45 weight percent or more loadings of MnO2 with bismuth incorporation lead to battery failure within 5 cycles.